Direct coupled balancer drive for floating lamp structure

J

Fi \\:4|_,E

I.

UÛVÛUÛUÛUÛUÛUÛUÛUÛ

flv|/Q2 o~_\\ m <3 J 2:\| E» \ ä T mm owmlw mm\ ow vä .x om J \8 2 _

1 DIRECT COUPLED BALANCER DRIVE FOR FLOATING LAMP STRUCTURE

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Patent Application Ser. No. 61/026,227 filed Feb. 5, 2008, U.S. Provisional Patent Application Ser. No. 61/055,993 filed May 25, 2008 and U.S. Provisional Patent Application Ser. No. 61/114,124 filed Nov. 13, 2008, the entire contents of all of which are incorporated herein by reference. This application is related to co-filed U.S. patent application Ser. No. 12/3 63,805 entitled “Arrangement Suitable for Driving Floating CCFL Based Backlight” and co-filed U.S. patent application Ser. No. 12/363,807 entitled “Balancing Arrangement with ReducedAmount of Balancing Transfonners”, the entire contents of each of which is incorporated herein by reference. This application is a continuation in part of pending U.S. patent application Ser. No. 11/937,693 filed Nov. 9, 2007.

BACKGROUND OF THE INVENTION

The present invention relates to the field of cold cathode fluorescent lamp based lighting and more particularly to an arrangement in which a balancing transfonner is supplied at the end of the lamp physically removed from the driving transfonner.

Fluorescent lamps are used in a number of applications including, without limitation, backlighting of display screens, televisions and monitors. One particular type of fluorescent lamp is a cold cathode fluorescent lamp (CCFL). Such lamps require a high starting voltage (typically on the order of 700 to 1,600 volts) for a short period of time to ior1ize a gas contained within the lamp tubes and fire or ignite the lamp. This starting voltage may be referred to herein as a strike voltage or striking voltage. After the gas in a CCFL is ionized and the lamp is fired, less voltage is needed to keep the lamp on.

In liquid crystal display (LCD) applications, a backlight is needed to illuminate the screen so as to make a visible display. Backlight systems in LCD or other applications typically include one or more CCFLs and an inverter system to provide both DC to AC power conversion and control of the lamp brightness. Even brightness across the panel and clean operation of inverters with low switching stresses, low EMI, and low switching losses is desirable.

The lamps are typically arranged with their longitudinal axis proceeding horizontally. In general, even brightness involves two dimensions: unifonn brightness in the vertical dimension, i.e. among the various lamps; and uniform brightness along the longitudinal axis of each of the various lamps in the horizontal dimension. Brightness unifonnity in the vertical dimension is largely dependent on matching the lamp currents which normally requires a certain type of balancing technique to maintain an even lamp current distribution. U.S. Pat. No. 7,242,147 issued Jul. 10, 2007 to Jin, entitled “Current Sharing Scheme for Multiple CCFL Lamp Operation”, the entire contents of which is incorporated herein by reference, is addressed to a ring balancer comprising a plurality of balancing transfonners which facilitate current sharing in a multi-lamp backlight system thus providing even lamp current distribution. The lamps are directly connected, either individually or in pairs, across the power supply.

Brightness unifonnity in the horizontal dimension is impacted by the existence of parasitic capacitance between the CCFLs and the chassis. As a result of the parasitic capaci

2

tance, leakage current exists along the length of the lamps and such leakage further results in diminishing brightness along the lamps’ longitudinal axis towards the cold end in a single ended drive architecture. The term single ended drive architecture refers to a backlight arrangement in which the high voltage drive power is applied from only one side of the lamp, which is usually called the ‘hot’ end, and the other side of the lamp is normally at ground potential and referred as the ‘cold’ end. With the increasing size of LCD televisions and monitors, increases in lamp length, wire length and operating voltage associated with the resultant large backlighting systems make the leakage effect more significant, and consequently unifonn horizontal brightness across lamps arranged in a single ended drive architecture is more diflicult to achieve. In order to obtain even horizontal brightness for each of the CCF lamps, i.e. that the lamps should not exhibit a light gradient along its longitudinal axis, energy has to be altematively driven into each end of the lamp. Thus, most large backlight inverter systems are configured to support ‘floating’ lamp structures, in which both lamp terminals are comiected to a high voltage driving source, with a 180° phase shift with respect to each other, and arranged to be floating in relation to the chassis ground plane.

As described above, a factor in achieving even brightness over a CCFL is the ability to symmetrically power the lamp altematively at both ends. This is more diflicult to achieve as the length of the lamp increases. Among the conventional inverter topologies, a phase shifted full-bridge topology and a resonant full-bridge topology are most commonly used for CCFL inverter applications because of their ability to produce symmetric lamp current waveforms and clean switching operations.

U.S. Pat. No. 7,187,139 issued Mar. 6, 2007 to Jin, entitled “Split Phase Inverters for CCFL Backlight System”, the entire contents of which is incorporated herein by reference, is addressed to an inverter arrangement in which the switching elements are split into two inverter arms that are deployed at separate tenninals of a floating lamp structure. Such a concept provides even brightness across the longitudinal dimension of the lamps with lower cost compared with the conventional approach of deploying a full bridge circuit at each end of the lamps, while maintaining the advantages of soft switching operation of the full bridge. Unfortunately, separate inverter circuits are still needed to develop driving power at both ends of the lamp.

SUMMARY OF THE INVENTION

Accordingly, in view of the discussion above, it is a principal object of the present embodiments to overcome at least some of the disadvantages of prior art. This is provided in certain embodiments by a backlighting arrangement in which a single balancing transformer is provided for each pair of lamps, the primary winding of each of the balancing transformers being arranged to be serially connected between the individual lamps of the respective pair of lamps. The secondary windings of the balancing transformers are connected in series, with a first end of the series arrangement being coupled to one lead of a driving transfonner arrangement providing a high voltage alternating current. In one embodiment the lamp pairs are constituted of linear lamps, one side of the linear lamps being connected to a driving transformer and physically located nearby, the balancing transfonners being connected at the side of the lamps removed from the driving transfonner.

Certain embodiments provide for a backlighting arrangement comprising: a driving transformer arrangement exhib

3

iting a first output lead and a second output lead, the first output lead and the second output lead of the driving transformer arrangement exhibiting opposing phases; a plurality of lamp pairs, each of the constituent lamps of the plurality of lamp pairs exhibiting a first electrical connection and a second electrical comiection; and a plurality of balancing transformers, each comprising a primary winding and a secondary winding magnetically coupled to the primary winding, and each associated with a particular one of the plurality of lamp pairs, the primary winding of each of the plurality of balancing transfonners being serially connected between the second electrical comiections of the constituent lamps of the associated lamp pair, wherein the secondary windings of the plurality of balancing transfonners are serially connected in phase, with a first end of the serially connected secondary windings of the balancing transfonners connected to the first output lead of the driving transformer arrangement, and the first electrical connection of at least one of the constituent lamps of each of the plurality of lamp pairs connected to the second output lead of the driving transformer arrangement.

Additional features and advantages of the invention will become apparent from the following drawings and description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections throughout.

With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for pmposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the accompanying drawings:

FIG. 1 illustrates a high level block diagram of an exemplary embodiment of a backlighting arrangement comprising a pair of driving transfonners whose primaries are serially connected, the ends of the serially connected primaries being coupled to opposing phases of a driver;

FIG. 2 illustrates a high level block diagram of an exemplary embodiment of a backlighting arrangement comprising a pair of driving transfonners whose primaries are connected in parallel, the ends of each of the parallel connected primaries being coupled to opposing phases of a driver;

FIG. 3 illustrates a high level block diagram of an exemplary embodiment of a backlighting arrangement comprising a driving transformer exhibiting a primary winding and two secondary windings, the ends of the primary winding being coupled to opposing phases of a driver; and

FIG. 4 illustrates a high level block diagram of an exemplary embodiment of a backlighting arrangement comprising a driving transfonner exhibiting a primary winding and a secondary winding, the ends of the primary winding being coupled to opposing phases of a driver.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present embodiments enable a backlighting arrangement in which a single balancing transformer is provided for

4

each pair of lamps, the primary winding of each of the balancing transformers being arranged to be serially connected between the individual lamps of the respective pair of lamps. The secondary windings of the balancing transfonners are connected in series, with a first end of the series arrangement being coupled to one lead of a driving transformer secondary winding arranged to provide a high voltage alternating current. In one embodiment the lamp pairs are constituted of linear lamps, one side of the linear lamps being connected to a driving transfonner and physically located nearby, the balancing transfonners being connected at the side of the lamps removed from the driving transfonner.

Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and tenninology employed herein is for the pmpose of description and should not be regarded as limiting. The term connected as used herein is not meant to be limited to a direct cormection, and the use of appropriate resistors, capacitors and inductors does not exceed the scope thereof.

FIG. 1 illustrates a high level block diagram of an exemplary backlighting arrangement 10 comprising: a driving transfonner arrangement 15 constituted of a pair of driving transfonners 20 whose primary windings are serially connected, the ends of the serially connected primary windings being coupled to opposing phases of a driver 30, denoted respectively 32 and 34. Backlighting arrangement 10 further comprises: a plurality a lamp pairs 40A . . . 40K, generally lamp pair 40, each constituted of a first lamp 50 and a second lamp 55; and a plurality of balancing transformers 80, each associated with a particular lamp pair 40. Each of first lamp 50 and second lamp 55 exhibits a respective first connection 60 and a respective second connection 70. Lamp pairs 40 are arranged to provide a backlight for a display 90. In certain embodiments, first lamp 50 and second lamp 55 are linear lamps, preferably of like constituency. In certain embodiments driver 30 is constituted of a DC/AC inverter. In one particular embodiment, lamp pairs 40 are floating in relation to a chassis, and driving transfonners 20 are arranged on only one side of lamp pairs 40 which are arranged in a parallel configuration.

Each balancing transformer 80 comprises a primary winding and a secondary winding magnetically coupled thereto. Each driving transfonner 20 comprises a primary winding and a secondary winding magnetically coupled thereto. Driving transfonner 20 is preferably a step up transfonner arranged to deliver a high voltage across the secondary winding responsive to a changing waveform appearing across its primary winding. Outputs 32 and 34 of driver 30 are preferably 180° out of phase with each other, thereby generating the high voltage across the secondary winding of driving transformer 20.

The secondary windings of the balancing transformers 80 are connected in series, and in phase, to fonn a serial string of secondary windings 85. A first end of the secondary winding of first driving transfonner 20 is connected to a first end of the secondary winding of balancing transformer 80 associated with lamp pair 40A, via a connection 100, thus being connected to one end of serial string of secondary windings 85. The second end of serial string of secondary windings 85, constituted of one end of the secondary winding of balancing transfonner 80 associatedwith lamp pair 40K, is connected to